Convert an ORP reading in mV to ppm by interpolating between two of your own paired ORP and tested chlorine readings, or find the ORP for a ppm.
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ORP To ppm Formula
The following formula can be used only when you have established an appropriate, application-specific linear calibration between ORP and a particular concentration (ppm) over a limited range (for example, via lab testing in the same water chemistry, pH, temperature, and sensor/reference setup). ORP itself is an electrochemical potential (mV) and does not have a universal conversion to ppm.
Variables:
- C is the interpolated concentration (ppm) for your specific analyte; C1 and C2 are independently tested calibration concentrations
- E is the measured or target ORP (mV); E1 and E2 are its paired calibration readings using the same reference electrode
- Both calibration pairs must describe the same water and unchanged chemistry, temperature and probe setup; use only values between the calibration endpoints
For linear interpolation use t = (E − E1)/(E2 − E1), then C = C1 + t(C2 − C1). The optional logarithmic assumption instead uses ln(C) = ln(C1) + t[ln(C2) − ln(C1)] and requires positive concentrations. Two points define either curve but do not validate it. Verify the relationship with additional independent measurements. Do not extrapolate, set a dose or infer water safety from this tool.
| ORP (mV) | PPM |
|---|---|
| 400 | 1.333 |
| 425 | 1.417 |
| 450 | 1.500 |
| 475 | 1.583 |
| 500 | 1.667 |
| 525 | 1.750 |
| 550 | 1.833 |
| 575 | 1.917 |
| 600 | 2.000 |
| 625 | 2.083 |
| 650 | 2.167 |
| 675 | 2.250 |
| 700 | 2.333 |
| 725 | 2.417 |
| 750 | 2.500 |
| 775 | 2.583 |
| 800 | 2.667 |
| 825 | 2.750 |
| 850 | 2.833 |
| 875 | 2.917 |
| * Rounded to 3 decimals. Arithmetic example only: linear interpolation between hypothetical pairs (0 mV, 0 ppm) and (900 mV, 3 ppm). These values are not a pool calibration. Actual ORP–PPM relationships depend on the redox chemistry, pH, temperature, and sensor/reference conditions. | |
What is ORP To ppm?
ORP (oxidation-reduction potential, also called redox potential) is a millivolt (mV) measurement that indicates how strongly a solution tends to oxidize or reduce other substances. PPM (parts per million) is a concentration unit for a specific chemical in water (for example, free chlorine, dissolved oxygen, or another analyte). There is no universal, chemistry-independent conversion from ORP to ppm; at best, ORP can sometimes be empirically correlated to the ppm of a particular oxidant/disinfectant residual under fixed conditions. Higher ORP often indicates more oxidizing conditions, but it does not uniquely determine “contaminant ppm,” and the direction/magnitude of any correlation depends on the system.
How to Calculate ORP To ppm?
The following steps describe interpolation between paired measurements. ORP alone does not determine concentration; confirm concentration with an appropriate chemical test.
- First, measure the ORP (oxidation-reduction potential) value (mV) with your ORP probe/meter.
- Next, enter two paired ORP and independently tested ppm readings for the same analyte and unchanged conditions. Select the interpolation assumption supported by your calibration data.
- Next, enter the measured or target ORP within the calibration interval, or switch to the inverse ORP task.
- Finally, calculate the estimated concentration in parts per million (ppm).
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge (assumes you have calibrated a linear relationship for your specific system).
ORP (oxidation-reduction potential) = 600 mV
Hypothetical calibration pairs: 0 mV at 0 ppm and 900 mV at 3 ppm. Linear interpolation at 600 mV gives 2 ppm. This is an arithmetic example, not measured pool data.
